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Big bird Giant prehistoric 'terror birds' looked so fierce that many palaeontologists assumed they were terrifying predators, but new research finds that the would-be carnivores were probably herbivores.

The terror bird, aka Gastornis, grew to nearly one-and-a-half metres tall. It lived between 55 to 40 million years ago in Europe and possessed a huge, sharp beak.

"The terror bird was thought to have used its huge beak to grab and break the neck of its prey, which is supported by biomechanical modelling of its bite force," says Thomas Tütken from the University of Bonn, who led the research.

"It lived after the dinosaurs became extinct and at a time when mammals were at an early stage of evolution and relatively small; thus, the terror bird was thought to have been a top predator at that time on land."

Wrong, according to the latest findings, presented by Tütken and his team at the Goldschmidt conference in Florence this week.

An early clue came by way of footprints likely left behind by an American cousin of Gastornis. The footprints do not show imprints of sharp claws, which would have been expected as tools to grapple prey. Today's raptors, for example, sport such sharp claws.

Another clue is more obvious — the bird's hefty size and build. Can you imagine Sesame Street's Big Bird (with a big beak) running swiftly after prey? All of that bulk would not make for a very swift hunter. Some researchers theorised that terror birds ambushed prey, but even that seems pretty far-fetched.

To further explore the possibilities, Tütken and colleagues took a geochemical approach. They analysed the fossilised bones of the birds, focusing on calcium isotope composition. Isotopes are atoms of the same element with different numbers of neutrons.

In prior experiments, the scientists determined that the calcium isotopic composition becomes "lighter" as it passes through the food chain. They tested the method first with herbivorous and carnivorous dinosaurs — including top predator T. rex — as well as mammals living today. For this latest study, they applied the method to terror bird bones housed at the Geiseltal collection at Martin-Luther University in Halle.

They discovered that the calcium isotope compositions of terror bird bones are similar to those of herbivorous mammals and dinosaurs, and not to carnivorous ones.

"Tooth enamel preserves original geochemical signatures much better than bone, but since Gastornis didn't have any teeth, we've had to work with their bones to do our calcium isotope assay," Tütken explains.

As for many scientific puzzles, the case isn't completely closed just yet.

"Because calcium is a major proportion of bone — around 40 per cent by weight — its composition is unlikely to have been affected much by fossilisation," he says.

"However, we want to be absolutely confident in our findings by analysing known herbivores and carnivores using fossilised bone from the same site and the same time period. This will give us an appropriate reference frame for the terror bird values."

Even if the food was just plant based, it had to have been large and tough, given the impressive beaks the birds evolved.